The American National Standards Institute (ANSI) T1.418-2000 standard sets forth specifications for delivering symmetrical digital-subscriber-line (DSL) service at T1 rates over one copper twisted-pair lines (also referred to as “high-speed digital-subscriber-line 2 (HDSL2) service”) and two copper twisted-pair telephone lines (also referred to “referred to as high-speed digital-subscriber-line 4 (HDSL4) service”).
The ANSI T1.418-2000 standard specifies that, at the time a HDSL2 or HDSL4 line is initialized, various transmitter settings are to be optimized for the operational environment that exists at that particular time. Examples of transmitter settings that are typically optimized when such a DSL link is initialized include transmitter precoder coefficients and transmit power. After such a DSL link is initialized, the operational environment in which the line operates typically changes over time. It may be the case that the operational environment in which the line operates changes in a manner that causes the transmitter settings established during initialization to be suboptimal. Such suboptimal transmitter settings may result in the line having relatively poor signal-to-noise ratio margins (for example, 0 decibels (dB) to 1 dB) and/or a relatively high bit error rate (for example, as high as 10−4). The performance of the line in such a situation may result in customer complaints and/or a request for service.
Other xDSL technologies (SHDSL, ADSL, VDSL, and their variants) also optimize some of the transmitter settings during initialization. Of particular note is the transmit power setting. Most xDSL standards provide for a way to set the transmit power during initialization, but have no way to adjust the transmit power once the line is in data mode (also referred to as showtime). Over time, the transmit power may not be sufficient to overcome the noise environment.
One approach to resolving such a situation is to retrain the line when the performance of the line falls below a predefined performance threshold. Such an approach, however, takes the line out of service while the line is being retrained. As a result, the predefined performance threshold is typically set sufficiently low to avoid frequently retraining the line. However, the line will typically experience significant performance degradation before the performance threshold for triggering a retrain operation is reached. It may be the case that the operational environment for the line is such that the line operates for a significant period of time with significant performance degradation that is not sufficient to trigger a retrain of the line.